Addition, during TiO2 surface treatment, of certain chemicals under specific conditions significantly enhance a number of pigment performance properties in paints. These properties include durability and ease-of dispersion. The chemicals added during surface treatment that improve pigment end-use performance include metal oxides, particularly aluminum oxides and silicon dioxide. The exact nature of these oxides may in turn be influenced by the conditions under which they are formed and the presence of other reagents during the precipitation process. The present invention provides an easy dispersing, high durability TiO2 pigment and method of making that pigment. Such super-durable TiO2 pigment with improved gloss retention (durability) and dispersibility (easy dispersing) is useful in a variety of exterior applications, including but not limited to powder, coil, automotive.
Some multi-purpose TiO2 pigment grades that are commercially available are treated with a coating of amorphous alumina. In these products there is no silica present as a surface treatment. Although these products have durability that is sufficient for architectural exterior coatings, they are not super-durable. In these products materials such as zirconia and tin are used to stabilize the amorphous alumina. The use of zirconia and tin makes these grades more expensive to manufacture and to use. The present invention uses citric acid to stabilize the amorphous alumina. The combination of silica and citric acid stabilized amorphous alumina on a pigment seems to be a new approach that provides performance and cost efficiencies.
U.S. Pat. No. 3,825,438 teaches a process to make a coated titanium dioxide pigment with one or more hydrous oxides in the presence of a polyhydric alcohol and/or a carboxylic acid. In this process, the alcohol or acid may be added at any time in the process prior to the completion of the deposition of the coating. The teachings in U.S. Pat. No. 3,825,438 solve the problem of pigment agglomeration on storage. A broad series of organic additives are taught in this patent to accomplish this desired result. A critical teaching is that the point at which the alcohol and/or acid are added in the process has no influence on the results achieved in the pigment product.
U.S. Pat. No. 5,340,393 teaches a method of making a non-agglomerated water insoluble inorganic particle having a dense, amorphous silica coating. Citric acid is identified as a possible optional charge stabilization dispersing acid that may be used in the process to prevent agglomeration of particles in the process. This patent offers no teaching as to the deposit of amorphous alumina or special properties resulting from the use of citric acid over other dispersants taught.
U.S. Pat. No. 5,730,796 teaches a process to make a durable pigment having a coating of cerium oxide, a coating of dense, amorphous silica and preferably an outer coating of crystalline (hydrous) alumina. It makes no teaching or suggestion with respect to the use of citric acid in deposition of silica or aluminia to enhance pigment properties.
U.S. Pat. No. 5,700,318 teaches an alumina coated inorganic pigment having a first coating of boehmite alumina, a second coating of amorphous alumina and a third coating of boehmite alumina. Adjustment of precipitation pH is taught as a means to produce the crystalline and amorphous alumina coatings. No teaching or suggestion is made with respect to the use of citric acid in this process.
U.S. Pat. No. 3,506,466 teaches the use of a salt of a water soluble alcohol amine and an easily water soluble oxycarboxylic acid to enhance dispersibility. The salt is added after wet treatment and before grinding the pigment.
U.S. Pat. No. 5,824,146 teaches a titanium dioxide pigment with amorphous alumina product in vapor phase oxidation of titanium tetrachloride in the presence of aluminum trichloride. The pigment is slurried using a citric acid dispersant.
U.S. Pat. No. 5,824,145 teaches mixtures of titanium dioxide pigment slurry and slurry of amorphous alumina hydroxide formed from mixing the two component slurries and using citric acid as a dispersing agent foe the mixed slurries. There is no wet treatment process disclosed in this patent.
U.S. Pat. Nos. 2,885,366 and 3,437,502 provide the basic teachings for making a durable titanium dioxide pigment having an amorphous silica coating followed by a coating of mixed crystalline alumina. Neither of these patents teach the use of citric acid during the wet treatment process used to deposit these treatments on the pigment surface.
The present invention relates to a method for making an easy-dispersing, high durability titanium dioxide pigment comprising the steps in sequence:
A. (1) heating a slurry of titanium dioxide particles to a temperature of from 85 to 100xc2x0 C.,
(2) adding citric acid as a solution in water to the slurry to form a mixture,
(3) adjusting the pH of the mixture to 10 or more,
(4) adding sufficient sodium silicate as a water solution to the mixture to deposit silica on the surface of the particles of from 1 to 3% based on the weight of the titanium dioxide particles in the slurry,
(5) neutralizing the slurry by addition of a mineral acid over the course of one hour, thereby forming a slurry of silica coated titanium dioxide particles; and
B. (1) adjusting the temperature of the slurry of silica coated titanium dioxide particles to a temperature of from 55 to 90xc2x0 C.,
(2) adding sufficient sodium aluminate as a water solution to the slurry in step B(1) and adjusting the pH of the mixture formed to from 5 to 9 by addition of a strong mineral acid to deposit aluminia as Al2O3 of from 1 to 4% by weight based on the weight of titanium dioxide particles present in the slurry of step A(1) on the surface of the silica coated particles, and digesting the resulting mixture for from 15 to 30 minutes.
The method of the present invention may be used as a batch or a continuous process. In practicing the method it is preferred that in step A(1) the slurry is heated to a temperature of from 90 to 95xc2x0 C. It is also preferred that citric acid be added to the slurry to a concentration based on the weight of the titanium dioxide particles of from 0.1 to 2%, and even more preferred that citric acid is added to a concentration of 0.3 to 0.5%.
The strong mineral acid used in the present invention is selected from the group consisting of nitric, hydrochloric and sulfuric acids. The most preferred acid to use when treating pigment made by the chloride process is hydrochloric acid, and the most preferred acid to use when treating pigment made by the sulfate process is sulfuric acid.
In the method of the present invention it is preferred in step B(2) that the addition of sodium aluminate solution and the pH adjustment by addition of mineral acid is done simultaneously.
The present process offers an improved method of making a titanium dioxide particles wherein the particles in a water slurry are wet treated with water solution of sodium silicate and sodium aluminate to form titanium dioxide having a first coating of silica followed by a second coating of aluminia wherein the improvement comprises adding citric acid to the titanium dioxide slurry before the addition of sodium silicate solution.
The present invention provides a product and a process for making that product in which TiO2 is coated sequentially in a wet treatment process with amorphous silica and amorphous alumina following procedures similar to those currently employed except that citric acid is added during the deposition process.
In a typical application of this invention, slurry of TiO2 is heated to a predetermined temperature and citric acid is added. The slurry is then pH adjusted to a target and sodium silicate is added. The pH is slowly decreased to neutral by addition of acid, after which the slurry is allowed to digest for a period of time, at the end of which the pigment particles are well coated with a contiguous layer of silica. The temperature of the slurry is then changed, if desired, and sodium aluminate and acid are then added together in such a way that the final pH after reagent addition is controlled to a predetermined value. The slurry is allowed to digest for a certain amount of time, then processed into a finished material by well-established methods.
The following outlines the operation limits and ranges of the present process. Step 1 is the silica deposition. Level of citric acid is typically 0.5% citric acid based on weight of untreated TiO2.
The concentration of TiO2 in the slurry ranges from 200 to 450 grams per liter, although lower levels are also possible. The temperature is optimally from 85 to 100xc2x0 C., although lower temperatures might also be effective.
The pre-silica deposition pH is typically above 10.0 although this may depend on the equipment used (lower pH is possible for continuous wet treatment). The optimal silica deposition weight is between 3 and 6% as SiO2 based on weight of untreated TiO2. However, improvements due to the invention are likely to be seen at any silica level.
Any strong mineral acid, including HCl, HNO3 and H2SO4 may be used for neutralization. The optimal acid addition time for batch process is 30 minutes per 1% SiO2 added. Longer times lead to better product at the expense of rate. Digestion time is typically 15 minutes per 1% silica. Shorter times lead to slightly worse product made at higher rate.
Step 2 alumina deposition: The initial temperature is optimally 55 to 90xc2x0 C., although lower temperatures might also be effective (or even more effective. However, this requires chilling of the slurry at the expense of energy and time). Aluminate amount is Optimal between 1 and 4% as Al2O3 based on weight of untreated TiO2. Lower levels will begin to degrade pigment performance properties (ease of dispersion; dispersion stability; gloss), as will higher levels (tint strength, hiding power, gloss). Any strong mineral acid, including HCl, HNO3, and H2SO4 may be used.
Post addition pH: Optimal is between 5 and 9. Higher values might lead to undesired alumina phase; lower values to incomplete deposition. Digestion time is typically 15 to 30 minutes. Short times might lead to incomplete mixing/reacting of ingredients; long times needlessly decrease rate through the process.
Pigment compositions of the present invention include from 1 to 4% amorphous alumina based on the weight of the untreated TiO2; from 3 to 6% silica based on the weight of the untreated TiO2 where the silica is in the form of a silica glass.
Pigments of the present invention provide durability equal to that of DuPont""s R-960 grade, but with dispersion qualities superior to an architectural pigment grade combined with gloss retention much improved over that of such durable grades as R-960. This combination of properties is reflected by the term super-durable as a description of pigment type.
Acid Solubility Test
10 mL 66% sulfuric acid is added to a test tube containing a magnetic stirring bar, and the tube is placed in an aluminum heating block and heated to 175xc2x0. A 0.2000 g pigment sample is added to the tube, and digested for 1 hour with stirring. At the conclusion of the digestion period, the tube containing the pigment sample is cooled by pouring the acid mixture into a beaker of ice, and the residue in the tube and beaker is washed with distilled water. The pigment residue in the tube and beaker is collected in a 100 mL volumetric flask, the volume made up to 100 mL by addition of distilled water, and the contents mixed thoroughly. The contents of the volumetric flask are then filtered, and 10 mL of the resulting filtrate are added to a 25 mL volumetric flask, to which 2 mL 20% hydrogen peroxide, and sufficient 10% sulfuric acid to make 25 mL, are added. The resulting solution is allowed to stand one hour. Absorbance of the solution is then read at 400 nm using a 10 mm cell path. Soluble TiO2 is determined by reference to a previously prepared spectrophotometric curve obtained by measurement of samples containing known quantities of dissolved TiO2.
Dispersibility Testing
The dispersibility of the TiO2 pigment made according to Example 2 was determined and compared to commercially available TiO2 pigments. Generally speaking, this determination was made by dispersing the pigments in an alkyd vehicle under two different controlled conditions that spanned a wide range of dispersion energies. The resulting dispersions were then let down with additional vehicle and tested for cleanness (number of undispersed particles) using a Hegman gauge. It is desirable that this number is minimal, and that low value is achieved under the mildest grind conditions. A more exact description of this procedure is provided in the text of Example 2.